This invention relates generally to rotary machines, and more particularly to a clearance measuring system for determining clearance distances between rotating rotary machine members.
At least some known rotary machines use capacitance probe-based clearance measurement systems to monitor rotatable member clearances. Specifically, one such measurement system used in determining turbine blade tip clearance measurement uses a frequency modulated (FM) capacitance probe. Another known system uses DC measurement techniques. FM systems are advantageous in that these systems may be less affected by gas ionization effects that may be present in gas turbines. Specifically, the capacitance tip clearance system measures the capacitance between the probe and the blade tip. The measured capacitance is then related to tip clearance using a pre-determined calibration factor in conjunction with the fundamental relationship for capacitance,       C    =                            E          r                ⁢                  E          o                ⁢        A            d        ,where Er represents the relative permittivity of the dielectric between the electrodes, Eo represents the permittivity of free space, A represents the electrode area, and d represents the electrode separation. In this case one electrode is the blade tip, the other is a probe mounted on the engine casing.
However, despite the advantages provided with FM systems, at low rotational speeds and zero speed, FM capacitance probe clearance measurement systems may be ineffective. Specifically, at low speeds, the clearance system accuracy decreases and the clearance output decreases to substantially zero at zero speed such that measurements are unreliable.